Head injury is a leading cause of accidental death and disability among children in the United States, resulting in over 100,000 hospitalizations every year. Studies have shown that children under the age of 14 are more likely to sustain head injuries than adults, and that children's head injuries are often more severe than those sustained by adults. In general, head injuries fall into two main categories--focal and diffuse. Focal injuries are limited to the area of impact, and include contusions, hematomas, lacerations and fractures. Diffuse brain injuries involve trauma to the neural and vascular elements of the brain at the microscopic level. The effect of such diffuse damage may vary from a completely reversible injury, such as a mild concussion, to prolonged coma and death.
Based on data from CPSC's National Electronic Injury Surveillance System (NEISS) an estimated 606,000 bicycle-related injuries were treated in U.S. hospital emergency rooms in 1994. In addition, about 1000 bicycle-related fatalities occur each year, according to the National Safety Council. A Consumer Product Safety Commission study of bicycle use and hazard patterns in 1993 indicated that almost one-third of bicycle injuries involve the head. Published data indicate that, in recent years, two-thirds of all bicycle-related deaths involved head injuries. Younger children are at particular risk for head injury. The Commission's data indicate that the injury risk for children under 15 was over 5 times the risk for older riders. About one-half of the bicycle-related injuries to children under age 10 involved head injuries, compared to about one-fifth of injuries to older riders. Children were also less likely to have been wearing a helmet at the time of a bicycle-related incident than were adults. The Commission's Bicycle Use Study found that about 18 percent of bicyclists wear helmets. Research has shown that helmets may reduce the risk of head injury to bicyclist by 85 percent, and the risk of brain injury by about 88 percent. Impact attenuation is one of the most important characteristics of a protective helmets for avoiding head injury.
Other activities, such as roller skating, in-line skating and skate boarding are typically conducted on the same types of surfaces as bicycling and can generate speeds similar to bicycling. Therefore, similar patterns of injury and benefits of helmet usage can be expected. Similar design considerations would apply for protective helmets for skating activities, in terms of impact attenuation. One difference between bicycling injuries and skating injuries is that, while 90 percent of bicycle-related head injuries occur on the front of the head, 80 percent of skating-related head injuries occur on the back of the head. Consequently, protective helmets for skating activities may have somewhat different design considerations in terms of coverage and location of protective padding. Protective helmets for aquatic activities, such as windsurfing, kayaking or waterskiing, have similar design considerations in terms of impact attenuation, with the additional requirement for moisture resistance during longterm immersion. Protective helmets for some activities, such as skiing or mountaineering, in addition to impact attenuation, have a need for a broad range of service temperatures.
The Children's Bicycle Helmet Safety Act of 1994 was signed into law in the U.S. on Jun. 16, 1994. Section 16 CFR 1203.3 of the proposed rule published pursuant to this act provides that bicycle helmets manufactured after Mar. 15, 1995 must conform to one of the following interim safety standards: The American National Standards Institute (ANSI) standard Z90.4-1984, the Snell Memorial Foundation standard B-90, B-90S, N-94 or B-95, the American Society for Testing and Materials (ASTM) F 1447, or Canadian Standards Association standard CAN/CSA-D113.2-M89. A revised proposed version of rule 16 CFR 1203 by the Consumer Product Safety Commission was published in the Federal Register on Dec. 6, 1995. The standard in proposed rule 16 CFR 1203 and each of the designated interim standards are incorporated herein by reference.
Integral to the proposed standard and each of the interim standards is a test for impact attenuation. The test measures the ability of the helmet to protect the head in a collision by securing the helmet on a headform with a weight of 5 kg for adult helmets or 3.9 kg for children's helmets and dropping the helmet/headform assembly from specified heights onto a fixed steel anvil. Three types of anvils are used for the test (flat, hemispherical, and "curbstone") representing types of surfaces encountered in actual riding conditions. Instrumentation within the headform records the acceleration during the headform's impact with the anvil in units of multiples of the acceleration due to gravity ("g"). Impact tests are performed on different helmets, each of which has been subjected to different environmental conditions. These environments are: ambient (room temperature), high temperature (117-127 .degree. F.), low temperature (3-9.degree. F.), and immersion in water for 4-24 hours.
Impacts are specified on a flat anvil from a height of 2 meters and on hemispherical and curbstone anvils from a height of 1.2 meters. In order for a helmet to be certified, the peak headform acceleration of any impact must not exceed 300 g under these test conditions. (An accepted industry standard is that test results of under 270 g allows sufficient safety margin to account for variations in the manufacturing of the helmets.)
Section 1203.11 of the proposed rule specifies the procedure for defining the area of the helmet that must provide impact protection. The original proposed rule also included an additional impact duration requirement that was eliminated from the revised standards, specifying maximum time limits of 6 milliseconds and 3 milliseconds are set for the allowable duration of the impact at the 150 g and 200 g levels, respectively. Some of the voluntary standards, e.g. Snell N-94, also provide for testing for multiple impacts at a single location on the helmet, but this requirement has not been included in the proposed standard.
Nearly one hundred percent of the protective helmets for bicycling currently on the market use expanded polystyrene foam (EPS) as a helmet liner to meet the impact attenuation requirements of the safety standards. The popularity of EPS as a protective helmet or helmet liner is due to a combination of multiple factors, including its impact attenuation capability, low cost, ease of manufacturing and light weight. However, EPS has a number of drawbacks as a protective helmet liner as well. The mechanism of impact attenuation exhibited by EPS, while highly effective, causes permanent and irreversible damage to the EPS material. The EPS material does not recover significantly after a serious impact, so that repeated impacts at the same location on the helmet do not receive the same degree of impact attenuation. This is not considered a serious drawback by many because, in accident sequences it is rarely observed that a helmet suffers two blows on the same site. Usually, the complex motions of the body during an accident mean that blows occur at different locations. What is considered a more serious problem is the deteriorated impact attenuation performance of the helmet in another accident at a later date.
Because the process of impact attenuation is destructive to the EPS helmet or helmet liner, manufacturers of EPS bicycle helmets recommend destroying and replacing the protective helmet after any serious impact or returning the helmet to the manufacturer. This recommendation is also reflected in the product labeling requirements of the proposed standards. This recommendation, if complied with, would help to assure proper head protection for bicycle riders. However, compliance by the consumer is voluntary, and many consumers, particularly children, may be reluctant to discard a helmet that appears to still be operative even though it has reduced impact attenuation. In addition, even relatively minor impacts to a helmet can cause microscopic cracks in the EPS material which can seriously deteriorate the impact attenuation performance of the helmet. Such damage can occur when the helmet is dropped or when something heavy is stacked on top of it in the trunk of a car. One of the characteristics of EPS that makes it prone to this kind of damage is that it has extremely low tensile strength. Any loading which places the EPS helmet or helmet liner in tension or bending is likely to cause damage to the EPS material that might compromise its impact attenuation properties. The lack of tensile strength in the EPS material also limits its usefulness for full coverage or wrap-around style helmets. Full coverage or wrap-around style helmets using EPS as an impact attenuation material must have an additional hard shell to support tensile or bending stresses that would damage the EPS helmet liner.
Environmental conditions can also deteriorate the impact attenuation performance of an EPS protective helmet. Moisture can penetrate the cell structure of the EPS material and deleteriously affect the protective performance of the helmet. Moisture exposure can happen from wearing the protective helmet while riding in the rain or even from the perspiration of the rider. Moisture sensitivity is a particular problem in helmets for use in aquatic activities, such as windsurfing, kayaking or waterskiing, where the helmet may be subject to repeated or prolonged immersion in water. High temperatures can also deteriorate the impact attenuation performance of an EPS protective helmet. Temperatures in a closed automobile in the summertime can sometimes exceed 130.degree. F. At these elevated temperatures, molding stresses from the EPS manufacturing process may warp the helmet and render it unusable. In addition, residual chemical blowing agents in the EPS may become reactive at elevated temperatures causing changes to the cell structure of the material which may affect its impact attenuation.
Another aspect of using EPS as an impact attenuation material in protective helmets is that the current safety standards may reflect the maximum protective performance possible from this material. Historically, the impact attenuation performance of EPS helmets has had to be improved to meet escalating safety standards based on public awareness of the need for better safety protection. In 1985, to conform with the Snell standards for impact attenuation, protective helmet liners were made with EPS material having a density of 4.5 to 5 pounds per cubic foot (pcf). In 1990, when the safety standards were raised, EPS material with a density of 5.5 to 6 pcf was needed to meet Snell standards for impact attenuation. Since adoption of the current safety standard, manufacturers have had to develop EPS materials with a density of 6.5 to 7 pcf to meet the new impact attenuation requirements. The newer, higher density EPS materials are harder to manufacture and further increases in the density may make the EPS too solid to be effective as an impact attenuation material. In addition, the nature of the EPS molding process precludes the possibility of manufacturing a dual density, laminated helmet of EPS. Current standards may represent the ultimate safety protection possible from EPS materials. Tightening safety standards in the future may actually exclude EPS as an impact attenuation material for protective helmets. To make further improvements in safety standards possible, new materials and construction methods for protective helmets will be needed.